1. Field of the Invention
The present invention relates to a multi-element planar array antenna which comprises a plurality of antenna elements arranged on a two-dimensional plane, and more particularly, to a multi-element planar array antenna which improves the polarization characteristics to facilitate the utilization of polarization components, and can be readily reconfigured into an active antenna by mounting a semiconductor device, IC (integrated) and the like thereon.
2. Description of the Related Arts
Planar antennas are widely used in for example, radio communications and satellite broadcasting in a microwave band and a millimeter band. Planar antennas are classified into a microstrip line type, a slot line type, and the like. Generally, the microstrip line planar antenna is often used because of a simple structure in a feed system, better radiation characteristics, and the like.
In recent years, a so-called multi-element array structure using a plurality of antenna elements has been employed with the intention of improving the antenna gains which is a challenge for the microstrip line planar antenna. As is well known, electromagnetic radiations include polarization components such as horizontal and vertical linear polarizations, and right-handed and left-handed circular polarizations. Many antennas making use of such polarization characteristics are widely used with the intention of sharing an antenna for transmission and reception, effectively utilizing the frequency resources, suppressing interference between transmission and reception, and the like.
FIGS. 1A to 1D are plan views respectively illustrating exemplary configurations of conventional planar antennas. Out of these planar antennas, those illustrated in FIGS. 1A, 1B and 1C are microstrip line planar antennas, while that illustrated in FIG. 1C is a slot line planar antenna. Each of these figures illustrates an exemplary configuration of a planar antenna having single antenna element 1 for producing a linear or a circular polarization.
The planar antenna illustrated in FIG. 1A is a microstrip line planar antenna for linear polarization which comprises square antenna element (i.e., circuit conductor) 1 and feed line 2 on one principal surface of substrate 3 made, for example, of a dielectric material. A ground conductor is disposed substantially over the entirety of the other principal surface of substrate 3. In this planar antenna, the antenna frequency (i.e., resonant frequency) is determined by the shape of antenna element 1, the dielectric coefficient of substrate 3, and the like. Also, in this planar antenna, a polarization plane of linear polarization for transmission and reception is set by a feeding direction in which feed line 2 is connected. Specifically, as indicated by arrows, a vertical polarization component can be transmitted and received when antenna element 1 is fed in the vertical direction (up-to-down direction in the figure), while a horizontal polarization component can be transmitted and received when antenna element 1 is fed in the horizontal direction (left-to-right direction in the figure).
The planar antenna illustrated in FIG. 1B is a microstrip line planar antenna having square antenna element 1 on one principal surface of substrate 3, similar to the one illustrated in FIG. 1A, but differs in that antenna element 1 is fed at two points so that it is adapted for use with a circular polarization. Specifically, feed line 2 is branched into two in the middle such that one of the branch lines is used as a vertical feed line while the other is used as a horizontal feed line. The vertical and horizontal feed lines differ in the electric length from each other by one-quarter wavelength. As a result, a vertical polarization component is out of phase from a horizontal polarization component by 90 degrees (xcfx80/2), so that these polarization components are combined into a circular polarization. Consequently, the resulting planar antenna is capable of transmitting and receiving a circular polarization. It should be noted that the planar antennas illustrated in FIGS. 1A and 1B each utilize a degeneration mode in antenna element 1.
The planar antenna illustrated in FIG. 1C is a microstrip line planar antenna for circular polarization, in which degeneration is released in antenna element 1 to feed antenna element 1 at one point. In this planar antenna, portions of antenna element 1 in a set of diagonal directions are cut away to release the degeneration so that resonance modes in two directions (vertical and horizontal directions) are out of phase by 90 degrees from each other at the operating frequency of the antenna, thereby providing the capabilities to transmit and receive a circular polarization.
FIG. 1D illustrates a slot line planar antenna for use with a circular polarization which releases degeneration in an antenna element. This planar antenna comprises antenna element 21 having a slot line instead of an antenna element in a microstrip line planar antenna. Antenna element 21 is rectangular in shape and released from the degeneration, thereby constituting a resonator at the antenna frequency. When antenna element 21 is fed at one corner thereof, resonance modes in the two directions are out of phase by 90 degrees from each other, similar to the foregoing, thereby providing the capabilities to transmit and receive a circular polarization.
The conventional microstrip line type and slot line type planar antennas described above can be shared for a horizontal polarization and a vertical polarization, and transmit and receive the circular polarization when they are provided with a single antenna element alone. However, these conventional planar antennas are problematic in configuring a multi-element planar array antenna comprised of a plurality of antenna elements arranged in a two-dimensional plane while maintaining the above functions of the planar antenna having a single antenna element.
Specifically, any of the planar antennas of the types illustrated in FIGS. 1A to 1D encounters difficulties, when it is configured as a multi-element array, in implementing connections of the feed line to respective antenna elements, i.e., a feeder circuit on the plane. For this reason, a multi-layer substrate, for example, should be used to implement a feeder circuit, in which case difficult designing is obliged for ensuring the same line lengths, for example, from a feed point, due to a requirement of exciting the respective antenna elements in phase.
Further, when the configuration illustrated in FIG. 1B is used for a circular polarization antenna, a phase difference feeder circuit is required for each antenna element for giving a phase difference of xcfx80/2. In addition, the planar antenna illustrated in FIG. 1C suffers from a narrow operating frequency range on principles. The planar antenna illustrated in FIG. 1D is similar in that it encounters difficulties in double use of both vertical and horizontal polarization components, and an adaptation for a two-dimensional planar array antenna using a circular polarization component.
As described above, the conventional planar antennas, whichever one is concerned, generally have a problem in the double use of polarizations, and the adaptation for a two-dimensional planar array antenna using a circular polarization component.
It is an object of the present invention to provide a multi-element planar array antenna which has a two-dimensional array structure that can use polarization components together and use a circular polarization.
The inventors diligently investigated the configuration of planar antennas, and perceived the transmission characteristics and line structures of microstrip lines and slot lines formed on both sides of a substrate made of a dielectric material or the like, and particularly perceived features of an anti-phase serial branch from the slot line to the microstrip line, and a circuit in which microstrip lines intersect each other, reaching the completion of the present invention.
Specifically, the object of the present invention is achieved by a multi-element planar array antenna which includes a substrate, a ground conductor formed on a first principal surface of the substrate, a first and a second slot line formed in the ground conductor, and intersecting each other, a first and a second microstrip line formed on a second principal surface of the substrate, and traversing the first slot line respectively at positions corresponding to both end sides of the first slot line, a third and a fourth microstrip line formed on the second principal surface, and traversing the second slot line respectively at positions corresponding to both end sides of the second slot line, and four antenna elements of a microstrip line type formed respectively in intersection regions between both end sides of the first and second microstrip lines and both end sides of the third and fourth microstrip lines, respectively, on the second principal surface. Each antenna element is arranged for excitation in two directions by connecting one of both ends of one of the first and second microstrip lines with one of both ends of one of the third and fourth microstrip lines. The two excitation directions of each antenna element are typically orthogonal to each other, and each antenna element is excited in phase.
In this multi-element planar array antenna, a feed point is typically at the intersection of the first and second slot lines. An excitation mode is selected for each antenna element by selecting at least two of four corners formed in the ground conductor at the intersection and applying a high frequency signal to the selected corners.
Specifically, in the present invention, the microstrip lines are routed on both end sides of a set of intersecting slot lines to traverse them, so that a high frequency signal in a balanced mode, propagating through the slot line, is converted to an unbalanced mode by the microstrip lines, and branched in series in opposite phase. The resulting high frequency signals propagate through the microstrip lines.
An excitation direction in each antenna element can be selected by selecting corners of the ground conductor constituting the intersection of the set of the slot lines at the position of the intersection, and applying a high frequency signal to the selected corners. For example, by selecting corners to apply a high frequency signal between ground conductors on both sides of the first slot line, the high frequency signal is converted to the unbalanced mode by the first and second microstrip lines, so that each antenna element is fed in a direction orthogonal to the direction in which the first slot line extends. Similarly, by selecting corners to apply a high frequency signal between ground conductors on both sides of the second slot line, each antenna element is fed in a direction orthogonal to the direction in which the second slot line extends. By thus selecting a feed mode at the intersection, one of the first and second slot lines can be excited, and an excitation direction can be selected for each antenna element. Thus, the multi-element planar array antenna can select one from linear polarizations in orthogonal directions as well as can use the linear polarizations together.
Further, as one pair of corners in a diagonal direction is selected from four corners at the intersection and applied with a high frequency signal, both slot lines are excited so that each antenna element is simultaneously fed from the two directions orthogonal to each other. As such, polarization components in the two directions are combined to provide a polarization component in an intermediate direction of the two directions. In addition, when the corners in the respective diagonal directions are formed in pairs, and each pair is applied with a high frequency signal at a different level, the polarization direction can be arbitrarily controlled to utilize any polarization component.
Moreover, the first and second slot lines are set such that their electric lengths differ from each other by xcfx80/2 as calculated in terms of phase difference. By applying a high frequency signal to one pair of corners in one diagonal direction at the intersection, a circular polarization can be transmitted and received. For example, a circular polarization can be generated by delaying a vertical excitation component in phase from a horizontal excitation component by xcfx80/2. In this event, electromagnetic radiations can be a right-handed circular polarization or a left-handed circular polarization depending on to which pair of corners positioned in the diagonal directions a high frequency signal is applied at the intersection. It is therefore possible to select a circular polarization, and again select a right-handed circular polarization or a left-handed circular polarization as well as to use the right-handed circular polarization together with left-handed circular polarization by simultaneously selecting the right-handed circular polarization and left-handed circular polarization, wherein, by way of example, the right-handed circular polarization component is transmitted while the left-handed circular polarization component is received, thereby readily implementing a multi-element planar array antenna capable of selecting one from orthogonal circular polarizations and using them together.
Moreover, in the present invention, a 16-element planar array antenna and planar antenna having a larger number of antenna elements can be configured by utilizing an in-phase parallel branch of slot lines from microstrip lines.
As appreciated from the foregoing, the present invention can readily implement a four-element planar array antenna which can use together linear polarizations such as a horizontal polarization component and a vertical polarization component and it can also readily implement double use of circular polarization having a right-handed polarization component and a left-handed polarization component. In addition, the present invention can readily implement multi-element planar array antenna having eight-elements, 16-elements, 64-elements and the like. The present invention can readily implement a multi-band planar array antenna by use of two frequencies.
Since the present invention utilizes the series branch from the slot lines to the microstrip lines, the antenna elements are complementary in excitation, consequently providing a planar antenna which excels in suppression of cross polarizations and circular polarization axial ratio characteristics. Further, the planar antenna structure of the present invention facilitates mounting of a functional circuit such as a semiconductor device, an integrated circuit, an IC chip and the like for the slot lines, and therefore is effective in providing an active planar array antenna, an adaptive active planar array antenna, and a smart planar array antenna.